CHAPTER ONE



Mirror, mirror....



	In August, 1939, in the tiny town of Piqua in northeast Ohio, an unmarried woman gave birth, slightly prematurely, to twin boys. The country as a whole was struggling through the last of the depression years, and times were still hard in Ohio, like most other places in prewar America. This poor mother, a recent immigrant, simply had no way of supporting two newborn infants, so, like many others who found themselves in her position, she asked the hospital to help her place the boys for adoption. These were healthy, handsome babies, and they were taken quickly; both were adopted within a few weeks, both by middle-class working families. One of the families was from Piqua itself; the other was from across the state, in Lima; in what would be the first of a string of improbable coincidences involving these twins, both families named their newly adopted son James. But, aside from these first few weeks in the same hospital, the two "Jimmies" would remain largely unaware of each other's existence, and they would not be brought together again for nearly forty years. Only then would they learn that they were genetically identical - that they were, in a sense, clones of one another. 
	Just a few years later, in a small city in western Michigan, another set of twin boys were born, again to an immigrant mother, but this time to one who was fortunate enough to have a husband and, even more fortunately, a husband with a job. It didn't pay much - he stitched and delivered burlap sacks to potato growers in the area - but it was enough to keep them all together. The twins came at the end of a string of four other children, and were seen as a mixed blessing for an already hard-strapped family. But they were welcomed; the family just squeezed a bit more tightly together in their modest house. Unlike the two Jimmies, these two boys would grow up in the same home, with exactly the same siblings and neighbors and pets, for nearly eighteen years. And the Michigan twins were different in another way: they were not identical, but rather fraternal. 
	These two sets of twins would never meet; they were never, in fact, aware of each other's existence. But they present those who study the development of human behavior with a natural experiment, an experiment of the type that could never be done with human subjects in a research laboratory. They have the potential to provide us with important insights into the role of genetics versus environment, of "nature vs. nurture," in shaping who we turn out to be in life. The two fraternal twins, Tommy and Bernie, were genetically distinct, yet reared in the same environment. The two Jimmies, although genetically identical, were raised in different environments, with different parents, siblings, neighbors and pets.
	One of the two Jimmies became aware through his adoptive mother that he had a twin brother, and eventually set out to find him. He was successful, and when the two were finally reunited at age 39, and began to swap childhood stories with one another, they were astounded at the number of similarities in their lives. Some were trivial, and almost certainly nothing more than monumental coincidences. For example, each had adoptive siblings named Larry and pet dogs named Toy. Both preferred Miller Lite beer, and smoked Salem cigarettes. Each had married a woman named Linda, divorced, and then married a woman named Betty. One named his firstborn son James Alan, the other James Allen.  Each loved stock car racing and hated baseball. 	
But as time went on, they would find additional, and from a biological point of view more meaningful, similarities in their separate early lives. Their personalities, as described by family members and as clearly indicated by standardized personality assessment tests, were remarkably similar. Each had developed sinus headaches at about age ten, a condition that eventually developed into recurring migraine headaches. Their descriptions of their symptoms, elicited by specialists, were virtually identical. Each had been good at math in school,  while each struggled with English; their overall scholastic performances were remarkably similar. Each had a fondness for woodworking, and each developed the habit of biting his fingernails.
	Tommy and Bernie, on the other hand, were as different as they could be from the very first day. Tommy was a peaceful baby, slept a great deal, and responded positively to being held and touched. Bernie was the opposite. He fussed or cried constantly; cuddling and caressing by family members didn't seem to console him. In a matter of just a few weeks, everyone began speaking of the twins in quite different terms. Tommy, by common consent, was "an angel;" Bernie was "difficult." Before long, it was obvious that the family interacted rather differently with each of the boys. The differences in their behavior continued to develop as the boys became toddlers and started interacting with other children in the neighborhood. Tommy seemed content to stay in the house and play with his toys; Bernie charged outside, where he would engage in play with other children. He seemed to have a penchant for rough-housing, and often came home bruised or cut. These differences carried through to the boys' early years at school. Tommy was adored by all his teachers; Bernie was sent home frequently because of his disruptive behavior in class. 
	As the two Jimmies moved into their young adult years, during which they had no contact, they continued developing similar habits and personality traits. They expressed themselves alike, and used similar slang phrases. Both suddenly put on about ten extra pounds at roughly the same point in life. In the years preceding their ultimate reunion, both encountered assorted stresses that led to chest pains and high blood pressure. Both had difficulty sleeping, and both were taking Valium for general nervousness. Both held clerical jobs, and each had developed a fascination with police work; each had become volunteer Sheriff's assistant in their respective communities.
	Tommy's and Bernie's lives took very different courses. Tommy was a middling student, but went on for two additional years after high school at the local community college. After a short stint in the military, part of which was spent in Vietnam, he entered a Catholic seminary and became a priest. Bernie dropped out of high school, got into a series of scrapes with the law, and has been in and out of prison a good deal of his adult life. His social interactions have remained extremely difficult, and he never married. In spite of their extraordinarily different personalities, they have stayed in close touch over the years.
The behavior of these fraternal twins highlights a subtle but very important point about the interaction of individuals with their environment. We commonly think of the impact of the environment on the individual, but for human beings this interaction is actually a two-way street. Although Tommy and Bernie grew up in exactly the same environment, they manipulated that environment differently. This was not necessarily a willful or even conscious manipulation on their part. These were two genetically quite different individuals, and some of their differences were expressed as personality differences that were immediately perceived by the people around them. Based in turn on their own personalities, those people responded in different ways to Tommy and Bernie. It was not the environment that made the one quarrelsome, and the other mild mannered. But their differences elicited markedly different responses from those around them, and in that sense they cannot be said to have grown up in exactly the same environment.
	We will talk a good deal about "environment" as we proceed through this book, because it is impossible to talk about the role of genes in behavior without taking the environment into account as well. So let us take just a moment to think about what we mean when we refer to the environment. For most animals, the environment means what we might call the physical or ecological environment; the natural surroundings in which the individual "behaves:" competes for resources to stay alive, to find a mate, and to produce offspring. These basic behaviors are the same for humans as for animals, but we pursue them in two quite different sorts of environments. We, too, function within the context of an ecological environment; we, too, must eat, stay warm, find food and a mate. But unlike animals, we also function within the context of a cultural environment. Culture consists of a wide range of abstract ideas, social customs, rituals, creative works, and institutions that are largely made possible by language. The cultural environment shapes human beings every bit as forcefully as the ecological environment; in fact, it can be argued that culture is now a more important factor in human genetic evolution than is the ecological environment. So as we proceed through the chapters that follow, we should always bear in mind not only the role of environment in determining behavior in a general sense, but also the unique role of the cultural environment in determining human behavior in particular. The interaction between our genetic selves and our cultural selves is, as we will see, very complex indeed.    
The biological nature of twins.	
	Twins have always fascinated us. They are a mystery, and so it is perhaps not surprising that they had become bound up in mythology long before they became an object of medical investigation. They are feared in some cultures, and revered in others. Twinning is much more common than live births would suggest. Although only about four sets of fraternal twins, and one set of identical twins, are produced per 1,000 live births in the United States, sophisticated sonographic studies of early human pregnancies suggest that as many as one in eight conceptions - and possibly more - produces multiple embryos. The vast majority of these are lost in the first few weeks of pregnancy, and under normal circumstances their existence is undetected by either the mother or her physician. 
The most intriguing twins - identical twins - are created when a single, fertilized egg splits into two parts shortly after fertilization. This can occur at several different stages in development; the later it occurs, the more similar the twins. Once fertilization has taken place, the egg starts dividing and begins its journey through the Fallopian tube toward the uterus. The immediate product of the union of a sperm and egg is called a zygote, and so twins arising from a single fertilization event are also called monozygotic twins. Once the zygote starts dividing, the developing cell mass is called an embryo; when the embryo is clearly recognizable as human (after about five weeks), it is referred to as a fetus.  
	Amazingly, for a number of rounds of cell division after an egg is fertilized, entire individuals can be formed from only a portion of the cells in a developing embryo. None of the cells in the evolving cell mass has yet become specialized, and all retain the potential to produce an entire individual. Physical splitting of the cell mass, or "partitioning" as it is sometimes called, can occur at various stages in the early stages of the gestational process. This part of the twinning process is not well understood. It is not at all clear why such a coherent cell mass would break apart or, once it did, why the parts would not simply rejoin, since embryonic cells are very sticky. Early mouse embryo cell masses can be readily separated in the laboratory, but great care has to be taken to prevent the separated parts from rejoining. The stable partitioning of an early embryonic cell mass is such an unlikely event that many obstetricians wonder if it might be a type of birth defect, albeit an entirely harmless one. 
	Very early partitioning (up to three or four days post-fertilization) of human embryos results in identical twins that have separate placentas. Twins arising from partitioning events occurring after about four days (roughly 70 percent of monozygotic twin pairs) usually share a common placenta. There is some evidence that identical twins developing in separate placentas may develop slightly differently, and there has been a lengthy - and largely unresolved - debate about how this might affect the future development of the individuals involved. On occasion, partitioning may occur up to as late as ten days post-conception, and in these cases separation may be only partially complete, leading to various degrees of the condition known as Siamese twins. All three types of twins -shared placenta, individual placentas, and Siamese - are monozygotic and genetically identical, but their different developmental patterns may result in subtle differences between the twins as adults.
	More than two genetically identical individuals may arise from a single fertilization event, although this is quite rare. The developing embryonic cell mass may split into three or even more parts, resulting in multiple monozygotic individuals. The Diane quintuplets, born in Canada in 1935, were genetically identical and thus monozygotic. It is also possible to have mixed multiple births; it is not at all unusual for quadruplets to consist of one pair of identical twins and one pair of fraternal twins, for example.
	We must introduce a note of caution here about use of the term "genetically identical" with regard to monozygotic twins. Although monozygotic twins start out life from a common genetic blueprint, the development of a fully grown adult from the early zygotic stage is not a perfectly controlled process. Potentially mutagenic errors are regularly detected and removed in the line of cells giving rise sperm or eggs in each generation, but this process is considerably less strict in production of those cells that form the soma (all of the rest of the cells in the body) during the development of a single individual. Normally, mutations that arise in somatic cells do not spread far in the body, because most somatic cells give rise to only a few progeny in their lifetimes, particularly in behavior-generating tissues such as the brain. Nevertheless, the accumulation of somatic errors throughout life is a potential source of genetic differences in otherwise identical twins.  
	Even in the development of two individuals from completely identical genetic blueprints, the developmental pathways might not be exactly the same. Particularly in development of the nervous system, which is at the center of all behavior, there is a good deal of randomness in the generation of varying portions of the brain and peripheral nerves. During embryonic and fetal life, newly forming nerve cells toss out fibers more or less randomly into their immediate vicinity. These will by chance form connections with other nerve cells or with nearby muscle cells. Nerve cells failing to make a connection die off; those that establish a connection retain that connection essentially for life. But even two genetically identical twins will grow slightly different patterns of nerve cell connections, and these differences will surely be a basis for some of the differences between monozygotic twins. Detailed analyses of the brains of monozygotic twins have in fact revealed small but potentially important neuroanatomical variations.   
	Unlike identical twins, fraternal twins are conceived when two separate sperm fertilize two separate eggs, and they are thus referred to as dizygotic twins. They never share the same placenta. The resulting embryos (called fetuses after about five weeks, when they begin to look distinctly human) share the same womb at the same time, but they are no more alike genetically than any two children born to the same parents through different birth events. That makes them far more alike genetically (fifty percent alike, on average) than two children selected from the population at random, but still rather far from complete genetic identity. Identical twins (with extremely rare exceptions) are always of the same sex; fraternal twins each have a random chance of being male or female.
	Until well into the present century, same-sex twins were judged to be identical or fraternal largely on the basis of appearance. Occasionally, fraternal twins may seem so alike that it would be easy to mistake them for identical twins. On rare occasions, identical twins may seem slightly different in appearance. Unless twins are of the opposite sex, they are now routinely tested for blood group proteins or DNA markers that allow doctors and parents to make an unambiguous determination of their genetic status.  
What twins can tell us about the role of genes in human personality
Scientists interested in the genetics of human behavior are interested first and foremost in variability. The question really is not whether genes underlie human behavior. Ultimately every aspect of the existence of every biological organism is determined by its genes; humans are no different in this respect. The real question is to what extent variability in the genes underlying behavior contribute to the variability in human behavior we see around us, and to what extent this variability is determined by differences in the environment - the home in which the individual was raised, churches and schools attended, and the community in which the individual lives and works throughout his or her life.
	The study of such questions in humans is accompanied by a number of restraints. In subsequent chapters, we will look at the role of genes in causing variations in behavior in a wide range of animal species, from the simplest single-cell organisms, through fruit flies and roundworms, and in mammals such as rats and mice. Our information will come from a variety of experiments that are simply not possible in humans. Laboratory animals can be selectively bred to reveal inheritance patterns from generation to generation. In many of these species, dozens of generations can be produced in a single year; humans require over a dozen years to produce a single generation. If we suspect that a particular form of a given gene causes a particular behavioral variation in an animal species, we can very often insert that gene variant into one of them to see just what it does. 
We cannot do any of these things in humans. We can only observe from the outside whatever it is that humans do naturally and of their own free will. Thus one of the oldest (and still used) methods of studying the role of genes in human behavior is to look for patterns of behavioral inheritance in families. Traditionally, this involved applying certain behavioral assessment tests to as many members of as many generations of as many families as possible, and then applying statistical tests to the results to determine whether the transmission of these traits seemed to be heritable. While this approach has been very important in spotting potentially heritable human traits, it has suffered from questions both about the validity of the behavioral assessment tests used, and of the statistical methods used to define heritability. In later chapters, we will see how modern molecular genetics has greatly improved the power of family lineage studies, but admittedly many questions remain.   
	The study of behavior in twins, and in children adopted into genetically unrelated families, has also greatly strengthened family lineage studies of the heritability of behavior. The basic strategy in twin studies, for any given variable behavior such as personality, is to look at the variability in that behavior when comparing pairs of monozygotic twins reared together or apart, as well dizygotic twin pairs reared together or apart, and to compare both of these with variability in the same trait among non-twinned biological and adopted siblings. Monozygotic twins reared apart provide additional controls for the influence of genes and environment on behavioral variability; they are essentially identical genetically, but they grow up in different cultural environments. Dizygotic twins reared together test the effect of different genetic constitutions in the same environment. 
The results of such studies must also be interpreted by statistical means, and to be meaningful, many pairs in each category must be tested. When such comparisons are carried out on a large enough scale it is possible, as we will see shortly, to dissect out genetic influences from environmental influences, based on the degree to which the subjects under study share common genetic backgrounds versus common environmental influences. In working out the relative contributions of genes and environment to the differences we see between individuals, geneticists refer to a fairly straightforward formula:
			V = G + Es + En 
This formula says simply that the variability (V) we see between two individuals should be accountable for by a combination of differences in their genes (G), plus differences in their environment (E). Environmental differences (whether cultural or ecological) can in turn be divided into those differences that are shared between two individuals (s), and those that are not shared (n). For example, two children raised in the same home would share certain environmental factors relating to parents and other family members, but would not share certain things in their external environment - different school experiences, for example, and different friends. Particularly when looking at variability involving monozygotic twins, we could also add an additional term to this equation to reflect the possible differences in the detailed aspects of fetal development discussed earlier.
We will use the term "intrascale correlation", or simply "correlation," when we discuss comparisons of various individuals with respect to a given trait. Correlation in this sense is a complex parameter, dependent on sophisticated statistical arguments. For our purposes it will mean simply the following: a correlation of 1.0 indicates that the performance between tested pairs, carried out with large numbers of pairs, is absolutely identical; a correlation of 0 indicates the performance between tested pairs was completely random. In reality, correlations of 1.0 and 0 do not occur. Because even the same individual tested on different days rarely has a correlation of more than 0.9 on most tests, a score of 1.0 between two different individuals would not be considered real, and when the correlations of large numbers of pairs are averaged, a score of 1.0 is simply not seen. Two randomly selected individuals would by chance show some degree of correlation on most tests, although the average correlation over many pairs would seldom exceed .05 - .10. Particularly when randomly selected individuals are tested for several variables simultaneously, the overall correlation usually falls below statistical significance, but for that reason we can never say it is zero.
The Minnesota twins study 
	The case histories of the two Jimmies, and of Tommy and Bernie, are both real life stories. They are more than simply anecdotal; each is one of hundreds of fully researched, well-documented stories that could be told. The details of each story vary, but enough of these studies have been carried out that we can begin to see rather clearly which aspects of human behavior are strongly influenced by genetics, and which by environment. The two Jimmies were the first set of  identical twins reared apart  that evolved into what has become the Minnesota Study of Twins Reared Apart. This program, which also studies identical twins reared together, as well as dizygotic twins reared together or apart, was founded by Dr. Thomas Bouchard at the Minneapolis campus of the University of Minnesota. Dr. Bouchard still directs the program, and has become one of the world's foremost authorities on the study of human twins. 
	The purpose of the Minnesota study is to document as thoroughly as possible the heritability and development over time of the physical, mental, and personality traits that make up human individuality. It is one of the largest and most comprehensive such programs in the world, but there are numerous others, both in this country and abroad. Several Scandinavian countries have twin studies that are considerably older than the Minnesota study, but with smaller numbers of subjects. In addition to having one of the largest assemblages of monozygotic and dizygotic twins (over 8,000), the Minnesota registry also has twins separated earlier, and living apart longer, than any other twin registry in the world. Included among these are over 120 pairs of monozygotic twins reared apart, more than in any other study. In the remainder of this chapter we will focus largely on results from the Minnesota study, bringing in comparisons from other studies where appropriate and useful.  
	Identical twins reared apart are identified for the Minnesota study by a variety of means. There was considerable publicity when the two Jimmies - now known as Jim Springer and Jim Lewis - were first reunited in February, 1979. The resulting media attention, laced with hints that the researchers would be interested in finding additional sets of twins reared apart, led to the rapid identification of a number of other suitable pairs of both monozygotic and dizygotic twins. Twins came to the study through self-referral, or were brought to the attention of the researchers through the mediation of physicians or other health and social welfare professionals. There are now more than one hundred thirty pairs of monozygotic twins reared apart in the Minnesota study, and several sets of monozygotic triplets as well. The twin pairs are of both sexes. Although twins are still occasionally added to the registry, the conditions (largely economic) that led to the separation of twins so frequently in the 1930s and 40s have changed, and the splitting apart of twin pairs is not as frequent as it once was. 
	Twins entered into the study are tested over a period of about fifty hours with a battery of tests to determine their genetic relatedness and medical condition, including electroencephalograms (EEGs) to measure brainwave patterns. They are also evaluated extensively to determine their general psychological status. Among the instruments used for the latter purpose are four personality trait inventories, three occupational interest assessments, and four different mental abilities tests. Extensive life history interviews are used to judge the similarities and differences in the home environment in the case of twins reared apart, and to probe their attitudes toward a variety of social, religious and philosophical issues. Twins are always tested and interviewed separately, but simultaneously, to avoid any possibility of inadvertent exchange of information. 
The power of twin analyses to establish a role for genes in human behavior can be seen in results from the Minnesota twin study on personality, published in several papers beginning in 1990. There is a general agreement among psychologists that most personalities can be defined according to where individuals place along five personality "axes" (Figure 1-1). Tommy and Bernie were at opposite ends of the "agreeableness" axis, for example; the descriptors at either end of this axis fit them almost exactly. These traits are assessed by a combination of written questionnaires, plus interviews of the subjects and of family members, all supervised by trained psychologists. 
Large numbers of monozygotic and dizygotic twin pairs were analyzed for individual components that make up the five major personality axes. The results for ten of the components of the Minnesota Multiphasic Personality Inventory are shown in Figure 1.2A for monozygotic and dizygotic twins reared apart. The likelihood that identical twins shared the same personality traits was considerably greater than the likelihood that fraternal twins would share these same traits in almost every case. The correlations were then averaged for all the personality components tested (Figure 1.2B) Monozygotic twins reared together had an average correlation of 0.46  Monozygotic twins reared apart showed a correlation of 0.45 for the same traits. What this tells us is that monozygotic twin pairs are as like each other in terms of personality whether they were reared in the same or different environments. Dizygotic (fraternal) twins reared apart showed an average correlation of 0.26, which is consistent with the fact that dizygotic twins are on average about half as alike genetically as monozygotic twins. Randomly selected individuals showed no statistically reliable correlation on personality tests. 
The correlation values from monozygotic and dizygotic twin studies have been used to calculate the contribution of heredity to personality. In virtually every area of human psychological development that we would ordinarily associate with personality, including those that impact strongly on our social interactions, it has been found that, on average, about fifty percent of the variation among individuals is related to genetic differences. The Minnesota researchers believe this figure may be closer to seventy percent, because the personality tests used, when applied to the same individual on different days, show a correlation of only about 0.8. So if an 0.8 correlation is taken as potentially representative of complete genetic identity, then the correlations for all of the twin pairs in these tests would actually be higher than the raw scores suggest.
If fifty percent of the variability we see in personality among unrelated individuals is heritable, and thus presumably due to genes, what explains the remaining fifty percent or so of this variability? Is it the environment? Undoubtedly so. But remember, environmental influences can be broken down into shared and nonshared experiences. Which of these is most important in determining variability in human personality? 
It is tempting to assume that the degree to which monozygotic twins reared apart differ in personality is due to the fact that they grew up in different environments, and thus that nonshared environmental factors account for these differences. This is undoubtedly true, but it may not be the whole story. Remember our fraternal twin pair, Tommy and Bernie. Reared in the same ecological and cultural environment, they manipulated it in quite different ways, eliciting different responses from the same family members, for example. One could imagine that if both boys went to the same museum together on the same day, one might be very engaged with what he saw there, whereas the other was bored and extracted little from the experience. In a room full of people, one twin might plunge in and become the life of the party, while the other sought out a dim corner where he might not be noticed. Genetically different individuals will also likely select out different sets of friends from the very same environment; peer groups are an important source of environmental influence, but they must be created from the environment by individual choice. 
Similarly, it is thought, identical twins must also interact with and manipulate their environments. They will select from different environmental options those things which accord most naturally with their inborn genetic predispositions, those things with which they are most comfortable, and which give them the greatest satisfaction. Twins who are verbally adept might select books, whereas twins who are aggressive and physically adept might choose sports. Both of these things would likely be present in the environments of monozygotic twins reared apart. Fearful twins reared apart may shut out many "over-challenging" experiences in their respective environments; outgoing twins reared apart might actively seek the very same experiences. Just as genetically different individuals may select different things from the same environment and treat them differently, so too may genetically identical twins reared apart select similar things from their different environments, and treat them in a similar way. 
Nevertheless, the most reasonable conclusion from the studies of monozygotic twins reared apart is that nonshared environmental differences, interacting with their shared genetic constitutions, are a major contributor to the differences we observe in their personalities. We can reasonably infer that the same is true of the population generally. What then about shared environmental experiences? To what extent do they contribute to the similarities and differences we see in siblings growing up in the same environment? Surprisingly, numerous studies of twins, and both biological and adopted siblings, have shown that shared home experiences have a minimal effect in shaping the personalities of children. For a number of personality traits, there may be a small effect of shared home environment on personality in children prior to their teen years, when they tend to mimic parents, older siblings and neighbors as part of the learning process, and as a way of interfacing with the world. But this effect almost completely disappears by the time the children pass through adolescence and leave home. The similarities in siblings reared together appears to derive mostly from their shared genetic inheritance, and not from the home environment. 
The idea that parents, through their own behavior and example, play a dominant role in forming their children's personalities is one of our most cherished beliefs. It is fundamental to our notions of how cultural values are transmitted from one generation to the next, and to parental responsibility for how children turn out as adults. But cultural transmission and personality development are simply two very different things. What the data tell us quite clearly is that as children begin to interact with the larger external world, particularly in their teen years, their own genetically determined personality factors come very much to the fore. It is this largely cultural environment of the larger society, with its larger palette of choices, its greater range of stresses, that appears to be involved in testing and strengthening some elements of personality, perhaps leaving others undeveloped at key stages of personality growth. The dominant environmental element in the cultural transformation of children is for the most part their (largely nonshared) interactions with their peers, in school and in neighborhood play1. 
Our current best guess is thus that genetic factors and nonshared environmental factors contribute about equally to the differences we see in human personality. The less alike two individuals are genetically, and the greater their differences in environmental experience, the less alike they will be in terms of personality. Conversely, while shared genetics can account for up to about half of the similarities we see in individuals, shared environmental factors contribute very little to these similarities.
Given that experience of the world is cumulative, we might expect that, to the extent that nonshared environmental experiences are a major factor in shaping human personality, over time the impact of genes on personality would decrease, and the apparent effect of the environment would increase. We might predict that the longer genetically identical twins lived apart, for example, the less like each other they would become. But this is not what we see. The degree of likeness, of concordance on personality tests, holds up remarkably well, even into the eighth and ninth decades of life. Some tests even suggest that identical twins grow more alike with time, whether reared apart or together. It is often noted that we become more "set in our ways" with time. Our "way", it seems, although modified by our environment, is determined to a substantial degree by our genetic inheritance. 
Genes and behavior
	The idea that at least some of the variability we see in human personality is heritable, and therefore genetically determined, would certainly come as no surprise to many animal breeders. For at least half a millennium or more, dogs have been bred specifically to reinforce certain personality traits. Some dogs, ranging in size from tiny terriers to massive pit bulls or Dobermans, have been bred for their aggressive nature. Others, such as collies or spaniels, faithfully transmit a docile, loving nature from generation to generation. Yet others have been bred to perform certain tasks related to hunting or managing flocks of animals. In the laboratory, rats and mice have been selectively bred for many generations to create strains that are fearful or aggressive. These strains pass on their personality differences each time they breed. No one seriously questions the role of genes in the formation of animal personality.
	Results such as those shown in Figure 1-2 suggest that genes also play an important role in determining human personality. On the other hand, what does personality have to do with human behavior, which is the subject of our inquiry? And how exactly might genes be involved in determining personality? Perhaps most importantly, what is the underlying biological significance of human behavior? And why is it so variable? 
	As we will see throughout the rest of this book, all behavior, in ourselves and in all other animals, consists of three basic components. First is the perception of something in the environment - we sense some sort of stimulus that catches our attention. Second, there is an integration of this perception with previous experience, e.g., memory. Third, we mount a reaction to the stimulus. That is a very stripped down definition of behavior, but such a definition will be useful as we look at the variability in behavior we see among different organisms of the same species. Within this framework, variability in the genes governing our perceptions, and transferring this information to our brains, will ultimately influence how we respond to things in the environment, whether those "things" are physically real, like a predator, or abstract, like a melody. Genes affecting the way we process the resulting signal - how our brain cells talk with each other and with the rest of the body - will also affect behavior. Finally, genetic differences affecting the brain's ability to direct a response to a given stimulus could certainly alter behavior between individuals. Notice that at all three levels, we are talking about the nervous system - the brain and its collateral tissues - as the target for genetic regulation of, and variability in, behavior. Naturally occurring differences between individuals in the genes regulating any of these processes may well explain differences in the way different people react to the same external situation.
	The twin studies tell us that variations in genes between individuals contribute significantly to variability in human behaviors such as personality, but they tell us nothing about the possible nature of these genes. We will examine a number of genes known to be involved in various components of behavior, in ourselves and in other living organisms, in subsequent chapters. There is a long tradition in genetics of identifying individual genes based on mutations that alter an organism in some observable way. Much of the early history of human genetics was devoted to looking for single-gene differences that could explain differences in things like behavior. But research carried out over the past fifty years has made it absolutely clear that single genes by themselves are rarely - if ever - responsible for any human behavior. Even subdomains of personality, like the components laid out in Figure 1-1, are far too complex to be explained by single genes. Everything we understand about behavior suggests it will best be explainable by the interaction of many genes, with each other and with the environment, through multiple dimensions of time and space, and in ways we may not even yet be aware of. 
On the other hand single genes, when defective, can often disrupt human personality. A single-gene defect that results in chronic pain, for example, could cause significant changes in an individual's behavior. Huntington's disease, which is caused by a single gene, begins with major personality disturbances. The single genes which, when disabled by mutation, are known to cause the highly heritable early onset forms of Alzheimer's disease certainly cause major  personality disruptions. However, the fact that a single defective gene can disrupt a given personality trait tells us only that that gene is involved in the trait under consideration, not that it is the only gene involved.   
	We will return time and again to twin studies as we examine other areas of human behavior, because such studies offer the clearest insight we have into the role of genetics as well as environment in all aspects of the behavior of human beings. Anthropocentrism is always a danger in biology, but it is probably safe to say that behavior is more complex in human beings than in any other animal, if for no other reason than that our systems for integrating signals from the environment - our brains - are more complex. Complex systems can sometimes be more easily understood by first developing an understanding of how simpler systems work, and that is one approach we will use throughout this book. Where then can we find the simplest behavior? And how will we define behavior in the simplest systems? These are important questions, because a clearer understanding of the evolution of behavior, from its earliest appearance and most primitive expression, will contribute a great deal to our understanding of behavior in ourselves. We begin this journey in the next chapter. 
1 This concept and the data underlying it have been admirably summarized by Judith Rich Harris in her book, "The Nurture Assumption: Why Children Turn Out The Way They Do."  The Free Press (Simon and Schuster), New York, 1998.
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07/05/99